Serial data buses send data bits on one line. Additional lines may be included for transmitting clock, handshaking, and reset signals that indicate, among other things, when the data line output has stabilized and is no longer switching from one state to another. Additionally, data buses use these lines to separate messages. The HPIB, a parallel data bus, has special lines to indicate the beginning and end of a message. Some serial data buses use a reset line to identify the beginning and end of message. Other serial data buses eliminate the reset lines by using a header/trailer scheme to identify new messages. One serial data bus combines the clock signal and data on one line and uses a separate reset line to identify the beginning and end of messages. Hardware is used to extract the clock signal. Once the clock signal is extracted it is used to extract the data. This scheme has the disadvantage of requiring extensive hardware to extract the clock signal from the data/clock line. Also this scheme requires a reset line.
A separate line for signaling the beginning of a new message has several disadvantages. In addition to the reset wire, hardware is needed to drive the wire, to send the information, and to receive the information. This additional hardware increases computer system cost and increases computer system failure rate. Using headers and trailers to identify the beginning and end of messages has the disadvantage of increasing the amount of time required to send a message and consuming bus bandwidth (where bandwidth is the maximum possible number of signal transitions per second). Headers and trailers have the further disadvantage of requiring the message to be encoded so that the header or trailer cannot occur in a message. Message encoding further decreases bus bandwidth.